1. Field of the Invention
The invention relates to the coking of coal, more specifically, to the preparation and use of coals of marginal coking quality in commercial coking facilities.
2. Description of the Prior Art
Suitable quality low, medium and high-volatile content bituminous coal are beginning to be in short supply and/or are too expensive to be used in the production of metallurgical-grade coke. Partial or total replacement of the premium-grade coals with less expensive, generally lower quality coals, usually results in coal blends that are marginal in coking quality and in coke that is lower in physical stability than that normally produced. It has been shown by those skilled in the art that partial agglomeration of marginal quality coking coal blends yields coke that is higher in physical stability than coke that is produced from the same coal blends, but without agglomerates. It is therefore possible, in some cases, to use partially-agglomerated, marginal quality coal blends in conventionally-charged coke ovens without severe sacrifies in the quality of the coke produced. The agglomerates used are typically similar in shape and size to the charcoal briquettes that are available for home recreational consumption. These briquettes are compressed from loose coal that is mixed with a binder. Many binders have been effectively used, including plain water, light oil, pitch and coal tar. The most effective of these binders has proved to be coal tar. However, coal tar is a valuable substance, being the basis for many modern useful chemicals. The economics of the utilization of coal tar dictates that it is too expensive to utilize to any great degree as a binder for coal agglomeration.
The solution to the economic problems associated with the use of coal tar binder has been largely overcome by the development of coal-tar-based solutions and suspensions which greatly reduce the percentage of coal tar required to bond the coal in the briquetting process. The physical integrity of the briquettes may be further enhanced by preheating the coal fines prior to briquetting. The result is an economically feasible briquette with good structural qualities.
Unfortunately, a serious problem does occur with some coal blends that are charged into the coke oven as a mixture of briquettes and non-agglomerated coal. Carbonization pressures on the walls of the coke oven are greatly increased. As is well known to those skilled in the art, carbonization pressures beyond certain limits produce a high degree of probability that the coke oven will be overstressed, causing rapid deterioration of the refractory therein and a shortening of the expected production life thereof. For this reason most untried coals or blends of coals are tested to determine carbonization pressure. Only those which display carbonization pressures within acceptable limits are used in conventional coke oven.
Tests were run on several commercially accepted blends of low to medium-volatile content coal mixed with high-volatile content coal, both with and without the addition of briquettes. The briquettes used in these tests were pillow-shaped with an average size of 1.8".times.1.3".times.0.8". Additional information concerning these briquettes is shown in Table I.
Table I __________________________________________________________________________ Briquette Data Apparent Bulk Moisture Tar Density, Density, Blend No. Composition Content % Binder % lbs/cu.ft. lbs per cu ft __________________________________________________________________________ 1 85% Pittsburgh Seam 15% Beckley Seam 2.5 4.0 76.0 42.0 2 85% Elkhorn Seam 15% Beckley Seam 1.8 4.1 70.2 40.9 3 85% Illinois No. 6 Seam 15% Pocahontas No. 3 Seam 4.7 3.8 72.6 43.5 __________________________________________________________________________
The apparent density was determined by measuring the volume of water displaced by a briquette of known weight. The bulk density was determined by measuring the weight of briquettes contained in a cubic-foot box.
Table II shows three typical coal blends used in conventional coking operations, their carbonization pressures without briquettes, with 20% briquettes, and with 40% briquettes by weight. The carbonization pressures shown in Table II are maximum wall pressures in pounds per square inch as measured using a Koppers Company, Inc. movable wall test oven and the standard procedures applicable thereto, both of which are well known to those skilled in the art.
Table II __________________________________________________________________________ Carbonization Pressures of Coal Blends Maximum Briquetts Bulk Density Wall Pressure Blend No. Composition % by wt. lbs per cu ft psi __________________________________________________________________________ 1 85% Pittsburgh Seam 0 53.5 1.21 15% Beckley Seam 20 57.5 1.77 40 57.9 2.43 2 85% Elkhorn Seam 0 53.2 1.68 15% Beckley Seam 20 57.0 2.15 40 57.0 2.55 3 85% Illinois No. 6 Seam 0 52.1 1.71 15% Pocahontas No. 3 Seam 20 57.1 2.14 40 59.1 2.53 __________________________________________________________________________
It was determined in the three coal blends tested, as shown in Table II, that without briquettes, the carbonization pressures were somewhat lower than those same blends exhibited when containing briquettes. When the mixtures contained 20% briquettes, the carbonization pressures increased 25-50%; and when the mixtures contained 40% briquettes, the carbonization pressures increased 50-100%. Thus the problem is crystallized: How is it possible to utilize partially-agglomerated coal blends of marginal coking quality coals that will yield coke of the highest possible physical stability and yet still not develop coke oven wall pressures beyond the limits that are detrimental to the productive life of the coke oven?
The results shown in Table II are exemplifications of many tests familiar to those skilled in the art. One of the parameters that appears from the many tests is that the lower the average volatile content of the coal blend, the higher the oven wall pressure. The average volatile content of the coal blend can be raised either by reducing the percentage of low to medium volatile coal in the blend or by including, into the blend, grades of coal more in the range of medium-volatile content, as distinguished from those in the range of low volatile content. Blends have been tested and found to be within the range of acceptable oven wall pressure, which range from 40% low and/or medium volatile coal blended with 60% high volatile coal to about 10% low and/or medium volatile coal blended with 90% high volatile coal. Many of these blends have been tested as including varying percentages of compacted coal fines, represented by briquettes, as well as without agglomerates. A correlation has developed to the extent that a second parameter can be stated. In such blends, the higher the percentage of briquette inclusions,